docs(event-handler): improve testing section for graphql

[to-mc]

Issue #, if available: #857

Description of changes:

Small improvements to "Testing Your Code" section in of graphql event handlers doc. Made example more relevant to a real application rather than borrowing our own example test, and added an example for async tests.

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

Breaking change checklist

RFC issue #:

• Migration process documented
• Implement warnings (if it can live side by side)

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View rendered docs/core/event_handler/appsync.md

[heitorlessa]

Looks much better. Only change is to make sure we handle a file descriptor correctly in the sample test

[codecov-commenter]

Codecov Report

Merging #996 (5cf6517) into develop (168ec48) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #996   +/-   ##
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  Coverage    99.96%   99.96%           
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  Files          119      119           
  Lines         5347     5347           
  Branches       610      610           
========================================
  Hits          5345     5345           
  Partials         2        2           

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[to-mc]

Suggested change added!